Evaluation of transcatheter heart valve biomaterials: Computational modeling using bovine and porcine pericardium

Effect of bioprosthetic leaflet anisotropy on stent dynamics of Transcatheter Aortic Valve Replacement devices

The assessment of stent fatigue in Transcatheter Aortic Valve Replacement (TAVR) systems is critical for the design of next-generation devices, both in vitro and in vivo. The mechanical properties of the bioprosthetic heart valves (BHVs) have a significant impact on the fatigue life of the metallic stent and thus must be taken into consideration when evaluating new TAVR device designs. This study aims to investigate the relationship between BHV anisotropic behaviour and the asymmetric deflections of the stent frame observed during in vitro testing. An explicit dynamics finite element model of the nitinol stent with attached bioprosthetic valve leaflets was developed to evaluate the deflections of the TAVR device under haemodynamic loading. Our results demonstrate that pericardium behaviour plays a dominant role in determining stent frame deflection. The anisotropic behaviour of the leaflets, resulting from collagen fibre orientation, affects the extent of deflection encountered by each commissure of the frame. This leads to asymmetric variation in frame deflection that can influence the overall fatigue life of the nitinol stent. This study highlights the importance of considering both the flexible nature of the metallic stent as well as the leaflet anisotropic behaviour in the design and fatigue assessment of TAVR systems.

Integrative lymph node-mimicking models created with biomaterials and computational tools to study the immune system

The lymph node (LN) is a vital organ of the lymphatic and immune system that enables timely detection, response, and clearance of harmful substances from the body. Each LN comprises of distinct substructures, which host a plethora of immune cell types working in tandem to coordinate complex innate and adaptive immune responses. An improved understanding of LN biology could facilitate treatment in LN-associated pathologies and immunotherapeutic interventions, yet at present, animal models, which often have poor physiological relevance, are the most popular experimental platforms. Emerging biomaterial engineering offers powerful alternatives, with the potential to circumvent limitations of animal models, for in-depth characterization and engineering of the lymphatic and adaptive immune system. In addition, mathematical and computational approaches, particularly in the current age of big data research, are reliable tools to verify and complement biomaterial works. In this review, we first discuss the importance of lymph node in immunity protection followed by recent advances using biomaterials to create in vitro/vivo LN-mimicking models to recreate the lymphoid tissue microstructure and microenvironment, as well as to describe the related immuno-functionality for biological investigation. We also explore the great potential of mathematical and computational models to serve as in silico supports. Furthermore, we suggest how both in vitro/vivo and in silico approaches can be integrated to strengthen basic patho-biological research, translational drug screening and clinical personalized therapies. We hope that this review will promote synergistic collaborations to accelerate progress of LN-mimicking systems to enhance understanding of immuno-complexity.

Experimental and computational models for tissue-engineered heart valves: a narrative review

Valvular heart disease is currently a common problem which causes high morbidity and mortality worldwide. Prosthetic valve replacements are widely needed to correct narrowing or backflow through the valvular orifice. Compared to mechanical valves and biological valves, tissue-engineered heart valves can be an ideal substitute because they have a low risk of thromboembolism and calcification, and the potential for remodelling, regeneration, and growth. In order to test the performance of these heart valves, various animal models and other models are needed to optimise the structure and function of tissue-engineered heart valves, which may provide a potential mechanism responsible for substantial enhancement in tissue-engineered heart valves. Choosing the appropriate model for evaluating the performance of the tissue-engineered valve is important, as different models have their own advantages and disadvantages. In this review, we summarise the current state-of-the-art animal models, bioreactors, and computational simulation models with the aim of creating more strategies for better development of tissue-engineered heart valves. This review provides an overview of major factors that influence the selection and design of a model for tissue-engineered heart valve. Continued efforts in improving and testing models for valve regeneration remain crucial in basic science and translational researches. Future research should focus on finding the right animal model and developing better in vitro testing systems for tissue-engineered heart valve.

Comparison on the properties of bovine pericardium and porcine pericardium used as leaflet materials of transcatheter heart valve

BACKGROUND

In order to obtain the smaller delivery diameter, porcine pericardium had been used as a substitute material of bovine pericardium for the leaflet materials of transcatheter heart valve (THV). However, the differences between them had not been fully studied. Therefore, this study compared the microstructure, biochemical and mechanical properties of two materials and hydrodynamics of THV made by the two materials in detail.

METHODS

In this study, firstly, the microstructure of pericardium was analyzed by staining and scanning electron microscope; secondly, the biochemical properties of pericardium after different processes were compared by heat shrinkage temperature test, free amino and carboxyl concentration test, enzyme degradation test, subcutaneous implantation calcification analysis in rats; finally, the mechanical properties were evaluated by uniaxial tensile test before and after the pericardium being crimped, and then, the hydrodynamics of THV was studied according to the ISO5840 standard.

RESULTS

Compared with bovine pericardium, after the same process, porcine pericardium showed a looser and tinier fiber bundle, a similar free carboxyl concentration, a lower resistance to enzyme degradation, a significantly lower calcification, bearing capacity and damage after being crimped, a better hydrodynamic and adaption with lower cardiac output and deformation of implantation position. Meanwhile the dehydration process of pericardium almost had preserved all the biochemical advantages of two materials.

CONCLUSION

In this study, porcine and bovine pericardium showed some significant differences in biochemical, mechanical properties and hydrodynamics. According to the results, it was presumed that the thinner porcine pericardium might be more suitable for THV of right heart system. Meanwhile, more attention should be taken for the calcification of THV made by the bovine pericardium.

Transcatheter Heart Valves: A Biomaterials Perspective

Heart valve disease is prevalent throughout the world, and the number of heart valve replacements is expected to increase rapidly in the coming years. Transcatheter heart valve replacement (THVR) provides a safe and minimally invasive means for heart valve replacement in high-risk patients. The latest clinical data demonstrates that THVR is a practical solution for low-risk patients. Despite these promising results, there is no long-term (>20 years) durability data on transcatheter heart valves (THVs), raising concerns about material degeneration and long-term performance. This review presents a detailed account of the materials development for THVRs. It provides a brief overview of THVR, the native valve properties, the criteria for an ideal THV, and how these devices are tested. A comprehensive review of materials and their applications in THVR, including how these materials are fabricated, prepared, and assembled into THVs is presented, followed by a discussion of current and future THVR biomaterial trends. The field of THVR is proliferating, and this review serves as a guide for understanding the development of THVs from a materials science and engineering perspective.

The Comparison of Different Constitutive Laws and Fiber Architectures for the Aortic Valve on Fluid-Structure Interaction Simulation

Built on the hybrid immersed boundary/finite element (IB/FE) method, fluid-structure interaction (FSI) simulations of aortic valve (AV) dynamics are performed with three different constitutive laws and two different fiber architectures for the AV leaflets. An idealized AV model is used and mounted in a straight tube, and a three-element Windkessel model is further attached to the aorta. After obtaining ex vivo biaxial tensile testing of porcine AV leaflets, we first determine the constitutive parameters of the selected three constitutive laws by matching the analytical stretch-stress relations derived from constitutive laws to the experimentally measured data. Both the average error and relevant R-squared value reveal that the anisotropic non-linear constitutive law with exponential terms for both the fiber and cross-fiber directions could be more suitable for characterizing the mechanical behaviors of the AV leaflets. We then thoroughly compare the simulation results from both structural mechanics and hemodynamics. Compared to the other two constitutive laws, the anisotropic non-linear constitutive law with exponential terms for both the fiber and cross-fiber directions shows the larger leaflet displacements at the opened state, the largest forward jet flow, the smaller regurgitant flow. We further analyze hemodynamic parameters of the six different cases, including the regurgitant fraction, the mean transvalvular pressure gradient, the effective orifice area, and the energy loss of the left ventricle. We find that the fiber architecture with body-fitted orientation shows better dynamic behaviors in the leaflets, especially with the constitutive law using exponential terms for both the fiber and cross-fiber directions. In conclusion, both constitutive laws and fiber architectures can affect AV dynamics. Our results further suggest that the strain energy function with exponential terms for both the fiber and cross-fiber directions could be more suitable for describing the AV leaflet mechanical behaviors. Future experimental studies are needed to identify competent constitutive laws for the AV leaflets and their associated fiber orientations with controlled experiments. Although limitations exist in the present AV model, our results provide important information for selecting appropriate constitutive laws and fiber architectures when modeling AV dynamics.

Mapping of bovine pericardium to enable a standardized acquirement of material for medical implants

OBJECTIVES

Bovine pericardium - native, fixed as well as decellularized - is one of the most common implant materials in modern cardiovascular surgery. Although used in everyday procedures, there are no recommendations in regard to which part of the pericardium to prefer. It was the aim of this study, to identify areas of the pericardium with consistent properties and high durability.

METHODS

Fresh bovine pericardia were collected from a local slaughterhouse. The native pericardia were analyzed at 140 spots in regard to thickness and fiber orientation. Based on these results, five promising areas were selected for further evaluation. The pericardia were decellularized with detergents (0.5% sodiumdesoxycholate/0.5% sodiumdodecylsulfate) and subsequently incubated in DNAse. The two investigation groups native und DC consisted of 20 samples each. The efficiency of the decellularization was evaluated by DNA quantification, as well as DAPI and H&E staining. Biomechanical properties were determined using uniaxial tensile tests. To evaluate the microstructure, scanning electron microscopy, Picrosirius Red- and Movat's Pentachrome staining were utilized. To assess the long-term durability, patches were tested in a high-cycle system for a duration equaling the stress of three months in-vivo. Commercially available, fixed pericardium patches served as control group.

RESULTS

Only a limited part of the pericardium showed a homogenous and usable thickness. The decellularization removed all cell nuclei, proven by negative DAPI and H&E staining, and also significantly reduced the DNA amount by 84%. The mechanical testing revealed that two investigated areas had an inconsistent tensile strength. Microscopical observations showed that the integrity of the extracellular matrix did not suffer by the decellularization procedure. During the long-term testing, most of the pericardia slowly lost tautness, though none of them got measurably damaged. Especially one area showed no decline of tensile strength after durability testing at all. Decellularized patches and fixed patches achieved comparable results in mechanical testing and microscopical evaluation after the durability testing.

CONCLUSION

We could clearly document significant, location-based differences within single pericardia. Only one area showed consistent properties and a high durability. We highly recommend taking this into account for future implant material selections.

Bovine Pericardium of High Fibre Dispersion Has High Fatigue Life and Increased Collagen Content; Potentially an Untapped Source of Heart Valve Leaflet Tissue

Bioprosthetic heart valves (BHVs) are implanted in aortic valve stenosis patients to replace the native, dysfunctional valve. Yet, the long-term performance of the glutaraldehyde-fixed bovine pericardium (GLBP) leaflets is known to reduce device durability. The aim of this study was to investigate a type of commercial-grade GLBP which has been over-looked in the literature to date; that of high collagen fibre dispersion (HD). Under uniaxial cyclic loading conditions, it was observed that the fatigue behaviour of HD GLBP was substantially equivalent to GLBP in which the fibres are highly aligned along the loading direction. It was also found that HD GLBP had a statistically significant 9.5% higher collagen content when compared to GLBP with highly aligned collagen fibres. The variability in diseased BHV delivery sites results in unpredictable and complex loading patterns across leaflets in vivo. This study presents the possibility of a shift from the traditional choice of circumferentially aligned GLBP leaflets, to that of high fibre dispersion arrangements. Characterised by its high fatigue life and increased collagen content, in addition to multiple fibre orientations, GLBP of high fibre dispersion may provide better patient outcomes under the multi-directional loading to which BHV leaflets are subjected in vivo.

Acellular pericardium: A naturally hierarchical, osteoconductive, and osteoinductive biomaterial for guided bone regeneration

There is great demand for an improved barrier membrane with osteogenic potential for guided bone regeneration (GBR). Natural acellular porcine pericardium (APP) is increasingly used in regenerative medicine as a kind of common extracellular matrix materials. This study aimed to investigate its potential application in GBR, especially its osteoconductive and osteoinductive properties. Bio-Gide (BG), a commercial collagen membrane, was set as the control group. APP samples were characterized by physicochemical analyses and their biological effects on human bone mesenchymal stem cells (hBMSCs) and human gingival fibroblasts (hGFs) were also examined. Additionally, the osteogenic potential of APP was tested on a bilateral critical-sized calvarial defect model. We discovered that the smooth surface of APP tended to recruit more hBMSCs. Moreover, promoted proliferation and osteogenic differentiation of hBMSCs was detected on this side of APP, with increased alkaline phosphatase activity and upregulated expression of bone-specific genes. Besides, the rough side of APP showed good biocompatibility and barrier function with hGFs. Histologic observation and analysis of calvarial defect healing over 4 weeks revealed enhanced bone regeneration under APP compared with BG and the control group. The results of this study indicate that APP is a potential osteoconductive and osteoinductive biomaterial for GBR.

The Impact of Self-Expandable Transcatheter Aortic Valve Replacement on Concomitant Functional Mitral Regurgitation: A Comprehensive Engineering Analysis

Background

Mitral regurgitation (MR) is present in a large proportion of patients who undergo transcatheter aortic valve replacement (TAVR). However, existing clinical data on the impact of TAVR on early post-procedural MR severity are contradictory. Using a comprehensive computational engineering methodology, this study aimed to evaluate quantitatively the structural and hemodynamic impact of TAVR on aortic-mitral continuity and MR severity in a rigorously developed and validated patient-specific left heart (LH) computer model with aortic stenosis and concomitant functional MR.

Methods

TAVR procedure was virtually simulated using a self-expandable valve (SEV) at three implantation heights. Pre- and post-TAVR LH dynamics as well as intra-operative biomechanics were analyzed.

Results

No significant differences in early MR improvement (<10%) were noted at the three implantation depths when compared to the pre-TAVR state. The high deployment model resulted in the highest stress in the native aortic leaflets, lowest stent-tissue contact force, highest aortic-mitral angle, and highest MR reduction for this patient case. When comparing SEV vs. balloon-expandable valve (BEV) performance at an optimal implantation height, the SEV gave a higher regurgitant volume ⋅ than the pre-TAVR model (40.49 vs 37.59 ml), while the BEV model gave the lowest regurgitant volume (33.84 vs 37.59 ml).

Conclusions

Contact force, aortic-mitral angle, and valve annuli compression were identified as possible mechanistic parameters that may suggest avenues for acute MR improvement. Albeit a single patient parametric study, it is our hope that such detailed engineering analysis could shed some light into the underlying biomechanical mechanisms of TAVR impact on MR.

Simulation of semilunar valve function: computer-aided design, 3D printing and flow assessment with MR

BACKGROUND

The structure of the valve leaflets and sinuses are crucial in supporting the proper function of the semilunar valve and ensuring leaflet durability. Therefore, an enhanced understanding of the structural characteristics of the semilunar valves is fundamental to the evaluation and staging of semilunar valve pathology, as well as the development of prosthetic or bioprosthetic valves. This paper illustrates the process of combining computer-aided design (CAD), 3D printing and flow assessment with 4-dimensional flow magnetic resonance imaging (MRI) to provide detailed assessment of the structural and hemodynamic characteristics of the normal semilunar valve.

METHODS

Previously published geometric data on the aortic valve was used to model the 'normal' tricuspid aortic valve with a CAD software package and 3D printed. An MRI compatible flow pump with the capacity to mimic physiological flows was connected to the phantom. A peak flow rate of 100 mL/s and heart rate of 60 beats per minute were used. MRI measurements included cine imaging, 2D and 4D phase-contrast imaging to assess valve motion, flow velocity and complex flow patterns.

RESULTS

Cine MRI data showed normal valve function and competency throughout the cardiac cycle in the 3D-printed phantom. Quantitative analysis of 4D Flow data showed net flow through 2D planes proximal and distal to the valve were very consistent (26.03 mL/s and 26.09 mL/s, respectively). Measurements of net flow value agreed closely with the flow waveform provided to the pump (27.74 mL/s), confirming 4D flow acquisition in relation to the pump output. Peak flow values proximal and distal to the valve were 78.4 mL/s and 63.3 mL/s, respectively. Particle traces of flow from 4D-phase contrast MRI data demonstrated flow through the valve into the ascending aorta and vortices within the aortic sinuses, which are expected during ventricular diastole.

CONCLUSION

In this proof of concept study, we have demonstrated the ability to generate physiological 3D-printed aortic valve phantoms and evaluate their function with cine- and 4D Flow MRI. This technology can work synergistically with promising tissue engineering research to develop optimal aortic valve replacements, which closely reproduces the complex function of the normal aortic valve.

The impact of balloon-expandable transcatheter aortic valve replacement on concomitant mitral regurgitation: a comprehensive computational analysis

The aortic and mitral valves function in a reciprocal interdependent fashion. However, the impact of transcatheter aortic valve replacement (TAVR) on the aortic-mitral continuity and severity of mitral regurgitation (MR) are poorly understood. In this study, a comprehensive engineering analysis was performed to investigate the impact of TAVR on MR severity and left heart dynamics in a retrospective patient case who harbours bicuspid aortic valve stenosis and concomitant functional MR. The TAVR procedure was computer simulated using a balloon-expandable valve, and the impact of three implantation heights on aortic-mitral coupling, MR severity and device performance were analysed. The accuracy and predictability of the computer modelling framework were validated with pre- and post-operative echo data. The highest deployment model resulted in higher stresses in the native leaflets, contact radial force and stent recoil, while the midway implantation model gave better haemodynamic performance and MR reduction in this patient case. Although the regurgitant volume decreased (less than 10%) for the three deployment configurations, no significant differences in MR severity improvement and mitral leaflet tethering were found. Acute improvement in MR was (i) due to the mechanical compression of the stent against the aortic-mitral curtain, (ii) due to an immediate drop in the ventricular pressure and transmitral pressure gradient. Albeit a single real clinical case, it is our hope that such detailed engineering computational analysis could shed light on the underlying biomechanical mechanisms of TAVR impact on MR.